Apparatus for controlling negative pressure in internal combustion engine

ABSTRACT

An improved apparatus for controlling brake force is disclosed. An engine selectively performs a stratified charge combustion and a uniform charge combustion. A brake booster is actuated by negative pressure an absolute value of which is greater than a predetermined magnitude. An electronic control unit actuates the throttle valve to apply the negative pressure to the brake booster when the magnitude relating to the pressure in the brake booster is smaller than the predetermined value. Engine condition is converted from the stratified charge combustion to the uniform charge combustion. A fuel injector directly injects the fuel into the cylinder to set the engine to perform the stratified charge combustion. A basic injection timing of the fuel injector is computed based on the operation state of the engine. The basic timing is advanced when the stratified charge combustion is performed and the negative pressure is applied to the brake booster.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatuses for controlling negativepressure in internal combustion engines. More particularly, the presentinvention pertains to apparatuses for controlling negative pressure ininternal combustion engines that are provided with brake boosters, whichuse negative pressure to improve braking force.

2. Description of the Related Art

Brake boosters have become widely used in vehicles to decrease therequired pressing force of the brake pedal. A typical brake booster usesnegative pressure, which is produced in an intake passage downstream ofa throttle valve, as a drive source. In other words, negative pressureis communicated to the brake booster through a communicating pipeconnected to the downstream side of the throttle valve. Negativepressure corresponding to the pressed amount of the brake pedal acts ona diaphragm, which is incorporated in the brake booster, and increasesthe braking force.

However, internal combustion engines such as diesel engines do notcontrol the amount of air intake during operation. Thus, it is difficultto produce negative pressure at the downstream side of the throttlevalve. In such cases, vacuum pumps are provided to produce negativepressure for the brake booster.

Japanese Unexamined Patent Publication No. 61-21831 describes anapparatus that produces negative pressure for the brake booster when thevacuum pump malfunctions. The apparatus slightly closes the throttlevalve to produce negative pressure at the downstream side of thethrottle valve. The negative pressure is communicated to the brakebooster.

However, the employment of a vacuum pump increases the engine load anddegrades the fuel efficiency.

Furthermore, in engines that perform stratified charge combustion,stoichiometric air-fuel mixture is supplied to the vicinity of anignition plug in a cylinder. The other portions of the cylinder areprovided with only air. Hence, the throttle valve is substantiallycompletely opened during normal running conditions. As a result,practically no negative pressure is produced at the downstream side ofthe throttle valve. This causes the negative pressure communicated tothe brake booster to be insufficient.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide animproved apparatus for controlling the vacuum pressure in an engine thatperforms stratified combustion and has a brake booster that uses vacuumpressure to guarantee braking force. Specifically, it is an objective ofthe present invention to provide an apparatus that prevents misfirescaused by rich air fuel ratio and stabilizes the combustion state of theengine when stratified combustion is performed and the brake booster isprovided with vacuum pressure from the intake passage.

To achieve the above objective, the present invention provides anapparatus for controlling the brake force of a vehicle movable based onrotation of an engine with a plurality of cylinders. Each of saidcylinders has a combustion chamber that receives fuel from a fuelinjector and air from an air intake passage. The air and fuel are mixedand combusted in the combustion chamber. The engine selectively performsa stratified charge combustion and a uniform charge combustion. Thestratified charge combustion mode is selected to increase at least theamount of the air supplied to the engine and improve a combusting stateof the engine. The apparatus includes a brake booster for increasingsaid brake force in accordance with negative pressure applied thereto. Arestricting means restricts airflow in the air intake passage togenerate the negative pressure that is supplied to the brake booster. Anactuating means actuates the restricting means to apply the negativepressure to the brake booster when the magnitude relating to thepressure in the brake booster is smaller than a predetermined value. Theactuating means converts a running condition of the engine from thestratified charge combustion to the uniform charge combustion. A fuelinjector directly injects the fuel into the cylinder to set the engineto perform the stratified charge combustion. A measuring means measuresthe magnitude relating to the pressure applied to the brake booster. Adetecting means detects the operation state of the engine. A computingmeans for computing a basic injection timing of the fuel injector basedon the detected operation state of the engine. A control means controlsthe fuel injector based on the computed basic injection timing. Thecontrol means includes means for advancing the basic injection timingwhen the stratified charge combustion is performed and the negativepressure is applied to the brake booster.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel areset forth with particularity in the appended claims. The invention,together with objects and advantages thereof, may best be understood byreference to the following description of the presently preferredembodiments together with the accompanying drawings in which:

FIG. 1 is a diagrammatic drawing showing an apparatus for controllingnegative pressure in an engine according to a first embodiment of thepresent invention;

FIG. 2 is an enlarged cross-sectional view showing the engine cylinder;

FIG. 3 is a schematic drawing showing the brake booster;

FIG. 4 is a flowchart illustrating the negative pressure control routineexecuted by the ECU;

FIG. 5 is a flowchart illustrating the negative pressure control routinethat continues from FIG. 4;

FIG. 6 is a flowchart illustrating the negative pressure control routinethat continues from FIGS. 4 and 5;

FIG. 7 is a table (map) showing the relationship between the closingcompensation angle and the value obtained by subtracting the brakebooster pressure value from the negative pressure value when terminatingstratified charge brake control;

FIG. 8 is a flowchart showing a main routine executed by the ECU;

FIG. 9 is table (map) showing the relationship between the throttleclosing angle and the fuel injection timing compensation angle;

FIG. 10 is a graph illustrating the relationship of the brake boosterpressure at low and high altitudes;

FIG. 11 is a graph showing the relationship between time and thethrottle closing angle;

FIG. 12 is a timing chart showing the relationship between time andair-fuel ratio and between time and the fuel injection signal.

FIG. 13 is a flowchart showing a part of a main routine according to asecond embodiment executed by the ECU;

FIG. 14 is a table (map) showing the relationship between the throttleclosing angle and the firing timing compensation angle;

FIG. 15 is a timing chart showing the relationship between time andair-fuel ratio, between time and the fuel injection signal and betweentime and firing timing according to the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of an apparatus for controlling negative pressure in aninternal combustion engine according to the present invention will nowbe described with reference to the drawings.

As shown in FIG. 1, an engine 1 is provided with, for example, fourcylinders 1a. The structure of the combustion chamber of each cylinder1a is shown in FIG. 2. As shown in these drawings, the engine 1 has acylinder block 2 that accommodates pistons. The pistons are reciprocatedin the cylinder block 2. A cylinder head 4 is arranged on top of thecylinder block 2. A combustion chamber 5 is defined between each pistonand the cylinder head 4. In this embodiment, four valves (first intakevalve 6a, second intake valve 6b, and two exhaust valves 8) are providedfor each cylinder 1a. The first intake valve 6a is provided with a firstintake port 7a while the second intake valve 6b is provided with asecond intake port 7b. Each exhaust valve 8 is provided with an exhaustport 9.

As shown in FIG. 2, the first intake port 7a is a helical port thatextends in a helical manner. The second port 7b extends in a straightmanner. Ignition plugs 10 are arranged at the middle of the cylinderhead 4. High voltage is applied to each ignition plug 10 by an ignitor12 though a distributor (not shown). The ignition timing of the ignitionplugs 10 is determined by the output timing of the high voltage sentfrom the ignitor 12. A fuel injection valve 11 is arranged near theinner wall of the cylinder head at the vicinity of each set of first andsecond intake valves 6a, 6b. The fuel injection valve 11 is used toinject fuel directly into the associated cylinder 1a and enables bothstratified charge combustion and uniform charge combustion.

As shown in FIG. 1, the first and second intake ports 7a, 7b of eachcylinder 1a are connected to a surge tank 16 by a first intake passage15a and a second intake passage 15b, which are defined in an intakemanifold 15. A swirl control valve 17 is arranged in each second intakepassage 15b. The swirl control valves 17 are connected to, for example,a step motor 19 by a common shaft 18. The step motor 19 is controlled bysignals sent from an electronic control unit (ECU) 30. The step motor 19may be replaced by an actuating member controlled by the negativepressure in the intake ports 7a, 7b.

The surge tank 16 is connected to an air cleaner 21 through an intakeduct 20. An electrically controlled throttle valve 23, which is openedand closed by a step motor 22, is arranged in the intake duct 20. TheECU 30 sends signals to drive the step motor 22 and open and close thethrottle valve 23. The throttle valve 23 adjusts the amount of intakeair that passes through the intake duct 20 and enters the combustionchambers 5. The throttle valve 23 also adjusts the negative pressureproduced in the intake duct 20.

A throttle sensor 25 is arranged in the vicinity of the throttle valve23 to detect the opening angle (throttle angle TA) of the valve 23. Theexhaust ports 9 of each cylinder 1a are connected to an exhaust manifold14. After combustion, the exhaust gas is sent to an exhaust pipe (notshown) through the exhaust manifold 14.

A conventional exhaust gas recirculation (EGR) mechanism 51 recirculatessome of the exhaust gas through an EGR passage 52. An EGR valve 53 isarranged in the EGR passage 52. The EGR passage 52 connects thedownstream side of the throttle valve 23 in the intake duct 20 to anexhaust duct. The EGR valve 53 includes a valve seat, a valve body, anda step motor (all of which are not shown). The opening area of the EGRvalve 53 is altered by causing the step motor to intermittently displacethe valve body with respect to the valve seat. When the EGR valve 53opens, some of the exhaust gas sent into the exhaust duct enters the EGRpassage 52. The gas is then drawn into the intake duct 20 via the EGRvalve 53. In other words, some of the exhaust gas is recirculated by theEGR mechanism 51 and returned to the air-fuel mixture. The recirculationamount of the exhaust gas is adjusted by the opening amount of the EGRvalve 53.

As shown in FIGS. 1 and 3, a brake booster 71 is provided to enhance thebraking force of the vehicle. The brake booster 71 increases thepressing force of the brake pedal 72. The pressing force is converted tohydraulic pressure and used to actuate brake actuators (not shown)provided for each wheel. The brake booster 71 is connected to thedownstream side of the throttle valve 23 in the intake duct 20 by aconnecting pipe 73 and actuated by the negative pressure produced in theduct 20. A check valve 74, which is opened by the negative pressureproduced in the intake duct 20, is provided in the connecting pipe 73(FIG. 3). The brake booster 71 includes a diaphragm, which serves as anactuating portion. One side of the diaphragm communicates with theatmosphere. The negative pressure produced in the intake duct 20 andcommunicated through the connecting pipe 73 acts on the other side ofthe diaphragm. A pressure sensor 63 is arranged in the connecting pipe73 to detect the pressure in the brake booster 71, or the brake boosterpressure PBK.

The ECU 30 is a digital computer provided with a random access memory(RAM) 32, a read only memory (ROM) 33, a central processing unit (CPU)34, which is a microprocessor, an input port 35, and an output port 36.A bidirectional bus 31 connects the RAM 32, the ROM 33, the CPU 34, theinput port 35, and the output port 36 to one another.

An acceleration pedal 24 is connected to an acceleration sensor 26A. Theacceleration sensor 26A generates voltage proportional to the degree ofdepression of the acceleration pedal 24. This enables the degree ofacceleration pedal depression ACCP to be detected. The voltage output bythe acceleration sensor 26A is input into the input port 35 by way of ananalog to digital (A/D) converter 37. The acceleration pedal 24 is alsoprovided with a complete closure switch 26B, which detects whether theacceleration pedal 24 is not pressed at all. The closure switch 26Boutputs a complete closure signal XIDL set at one when the accelerationpedal 24 is not pressed at all and outputs a complete closure signalXIDL set at zero when the acceleration pedal 24 is pressed. The outputvoltage of the closure switch 26B is also input into the input port 35.

A top dead center position sensor 27 generates an output pulse when, forexample, the piston in the first cylinder 1a reaches the top dead centerposition. The output pulse is input into the input port 35. A crankangle sensor 28 generates an output pulse each time a crankshaft of theengine 1 is rotated by a crank angle CA of 30 degrees. The output pulsesent from the crank angle sensor 27 is input into the input port 35. TheCPU 34 reads the output pulses of the top dead center position sensor 27and the crank angle sensor 28 to compute the engine speed NE.

The rotational angle of the shaft 18 is detected by a swirl controlvalve sensor 29 to measure the opening area of the swirl control valves17. The signal output of the swirl control valve sensor 29 is input intothe input port 35 by way of an A/D converter 37.

The throttle sensor 25 detects the throttle angle TA. The signal outputof the throttle sensor 25 is input into the input port 35 by way of anA/D converter 37.

An intake pressure sensor 61 is provided to detect the pressure in thesurge tank 16 (intake pressure PiM). The intake pressure PiM detected bythe intake pressure sensor 61 when the engine 1 is started issubstantially equal to the atmospheric pressure PA. Thus, the intakepressure sensor 61 also detects atmospheric pressure.

A coolant temperature sensor 62 is provided to detect the temperature ofthe engine coolant (coolant temperature THW). A vehicle speed sensor 64is provided to detect the speed of the vehicle (vehicle speed SPD). Thesignal outputs of the sensors 61, 62, 64 are input into the input port35 by way of A/D converters 37. The signal output of the pressure sensor63 is also input into the input port 35 by way of an A/D converter 37.

The running condition of the engine 1 is detected by the throttle sensor25, the acceleration sensor 26A, the complete closure switch 26B, thetop dead center position sensor 27, the crank angle sensor 28, the swirlcontrol valve sensor 29, the intake pressure sensor 61, the coolanttemperature sensor 62, the pressure sensor 63, and the vehicle speedsensor 64.

The output port 36 is connected to the fuel injection valves 11, thestep motors 19, 22, the ignitor 12, and the EGR valve 53 (step motor) byway of drive circuits 38. The ECU 30 optimally controls the fuelinjection valves 11, the step motors 19, 22, the ignitor 12 (ignitionplugs 10), and the EGR valve 53 with control programs stored in the ROM33 based on signals sent from the sensors 25-29, 61-64.

Control programs stored in the apparatus for controlling negativepressure in the engine 1 will now be described with reference to theflowcharts shown in FIGS. 4-6. A routine executed to control thenegative pressure communicated to the brake booster 71 by controllingthe throttle valve 23 (the step motor 22) is illustrated in FIGS. 4-6.This routine is an interrupt executed by the ECU 30 at predeterminedcrank angle intervals.

When entering the routine, the ECU 30 first determines whether theengine 1 is presently performing stratified charge combustion in step100. If stratified charge combustion is not being performed, the ECU 30determines that the engine 1 is presently performing uniform chargecombustion. This indicates that problems related with negative pressureare unlikely to occur. In this case, the ECU 30 proceeds to step 112.

In step 112, the ECU 30 computes the basic throttle angle TRTB from thepresent detecting signals (the degree of acceleration pedal depressionACCP, the engine speed NE, and other parameters). The ECU 30 refers to amap (not shown) to compute the basic throttle angle TRTB.

The ECU 30 proceeds to step 113 and sets the final target throttleangle, or throttle opening area TRT, by subtracting the present throttleclosing angle TRTCBK from the basic throttle angle TRTB. The ECU 30 thentemporarily terminates subsequent processing. When the ECU 30 jumps fromstep 100 to step 112, the value of the throttle closing angle TRTCBK isset at zero. Thus, the basic throttle angle TRTB is set equal to thefinal target throttle opening area TRT.

In step 100, if it is determined that the engine 1 is performingstratified charge combustion, the ECU 30 proceeds to step 101. At step101, the ECU 30 subtracts the most recent brake booster pressure PBK,which is detected by the pressure sensor 63, from the atmosphericpressure PA to obtain the pressure difference DPBK.

In step 102, the ECU 30 determines whether the present vehicle speed SPDis equal to or higher than a predetermined speed (e.g., 20 km/h). If thevehicle speed SPD is lower than the predetermined speed, the ECU 30continues the stratified charge combustion mode and proceeds to step 103to execute the throttle angle control (stratified charge brake control).

In step 103, the ECU 30 determines whether the flag XBKIDL thatindicates the execution of the stratified charge brake control is set atone. The execution flag XBKIDL is set at one when producing negativepressure while performing the stratified charge combustion mode. If theexecution flag XBKIDL is set at zero, that is, if the stratified chargecontrol is not in process, the ECU 30 proceeds to step 104.

In step 104, the ECU 30 determines whether the present pressuredifference DPBK is smaller than a predetermined negative pressure valuetKPBLK (e.g., 300 mmHg), which initiates the stratified charge brakecontrol. If the pressure difference DPBK is smaller than the negativepressure value tKPBLK, the ECU 30 proceeds to step 105.

In step 105, the ECU 30 sets the execution flag XBKIDL to one to enterthe stratified charge brake control mode. The ECU 30 then proceeds tostep 106 and computes the closing compensation angle a. To obtain theclosing compensation value a, the ECU 30 refers to a map such as thatshown in FIG. 7. In the map, the closing compensation angles a areindicated in correspondence with values that are obtained by subtractingthe value of the pressure difference DPBK from the target negativepressure value tKPBKO (e.g., 350 mmHg). If the pressure difference DPBKis much smaller than the predetermined negative pressure value tKPBKO(i.e., if the subtracted value is large), the closing compensation anglea is set at a large value to increase the closing speed of the throttlevalve 23. On the contrary, the closing compensation angle a is set at asmall value to decrease the closing speed of the throttle valve 23 whenthe pressure difference DPBK approaches the predetermined negativepressure value tKPBKO (i.e., when the subtracted value is small).

In step 107, the ECU 30 renews the throttle closing angle TRTCBK to avalue obtained by adding the present closing angle compensation value ato the throttle closing angle TRTCBK of the previous cycle and thenproceeds to step 112. In step 112, the ECU 30 computes the basicthrottle angle TRTB. Then, in step 113, the ECU 30 sets the final targetthrottle opening area TRT by subtracting the present throttle closingangle TRTCBK from the basic throttle angle TRTB. Afterward, the ECU 30temporarily terminates subsequent processing. Accordingly, if the ECU 30carries out steps 103 to 107, the increasing value obtained bysubtracting the throttle closing angle TRTCBK is set as the final targetthrottle opening area TRT.

In step 104, if the pressure difference DPBK is equal to or greater thanthe negative pressure value tKPBLK that initiates the stratified chargebrake control, the ECU 30 jumps to step 112. In this case, stratifiedcharge brake control is not executed.

If the execution flag XBKIDL is set at one in step 103, the ECU 30proceeds to step 108 and determines whether the pressure difference DPBKexceeds the negative pressure value tKPBKO that terminates thestratified charge brake control. If it is determined that the pressuredifference DPBK does not exceed the negative pressure value tKPBKO, theECU 30 proceeds to step 106. The ECU 30 carries out steps 106, 107 andthen proceeds to step 112 to compute the basic throttle angle TRTB.Subsequently, in step 113, the ECU 30 sets the final target throttleopening area TRT to a value obtained by subtracting the present throttleclosing angle TRTCBK from the basic throttle angle TRTB. Afterward, theECU 30 temporarily terminates subsequent processing. Accordingly, inthis case, the value obtained by subtracting the presently increasingthrottle closing angle TRTCBK is set as the final target throttleopening area TRT.

If it is determined that the pressure difference DPBK exceeds thenegative pressure value tKPBKO in step 108, the ECU 30 proceeds to step109 to decrease the throttle closing angle TRTCBK (and increase thetarget throttle opening area TRT). At step 109, the ECU 30 renews thethrottle closing angle TRTCBK to a value obtained by subtracting apredetermined closing angle compensation value b (b is a constant value)from the throttle closing angle TRTCBK of the previous cycle.

In step 110, the ECU 30 determines whether the throttle closing angleTRTCBK corresponds to a value of zero. If it is determined that thethrottle closing angle TRTCBK does not correspond to a value of zero,the ECU 30 proceeds to step 112 to compute the basic throttle angleTRTB. Subsequently, in step 113, the ECU 30 sets the final targetthrottle opening area TRT to a value obtained by subtracting the presentthrottle closing angle TRTCBK from the basic throttle angle TRTB.Afterward, the ECU 30 temporarily terminates subsequent processing.Accordingly, in this case, the value obtained by subtracting thepresently decreasing value of the difference between the throttleclosing angle TRTCBK and the basic throttle angle TRTB is set as thefinal target throttle opening area TRT.

If the throttle closing angle TRTCBK corresponds to a value of zero instep 110, the ECU 30 proceeds to step 111. At step 111, the ECU 30 setsthe execution flag XBKIDL to zero to terminate the stratified chargebrake control mode. The ECU 30 then carries out steps 112, 113 andtemporarily terminates subsequent processing. When the ECU 30 proceedsfrom step 111 to step 112, the value of the throttle closing angleTRTCBK is set at zero. Thus, the basic throttle angle TRTB is set equalto the final target throttle opening area TRT.

In step 102, if the present vehicle speed SPD is equal to or higher thanthe predetermined speed, the ECU 30 proceeds to step 114 to temporarilyperform uniform charge combustion while executing throttle angle control(uniform charge combustion brake control).

In step 114, the ECU 30 determines whether the flag XBKDJ that indicatesexecution of the uniform charge combustion brake control is set at one.The execution flag XBKDJ is set at one when negative pressure isguaranteed by the performance of the uniform charge combustion. Ifdetermined that the execution flag XBKDJ is not set at one but set atzero, the ECU 30 proceeds to step 115.

In step 115, the ECU 30 determines whether the present pressuredifference DPBK is smaller than the negative pressure value tKPBKLS atwhich the uniform charge brake control is initiated (e.g., 300 mmHg). Ifdetermined that the pressure difference DPBK is equal to or greater thanthe negative pressure value tKPBKLS, the ECU 30 jumps to step 112. Inthis case, the uniform charge combustion brake control is not carriedout.

If the pressure difference DPBK is smaller than the negative pressurevalue tKPBKLS, the ECU 30 determines that negative pressure isinsufficient and proceeds to step 116. At step 116, the ECU 30 sets theexecution flag XBKDJ at one to execute uniform charge combustioncontrol. The ECU 30 then proceeds to step 117 and temporarily switchesto the uniform charge combustion mode. During stratified chargecombustion, either the stratified charge combustion mode or the uniformcharge combustion mode is performed. The stratified charge combustioncontrol is usually performed. However, uniform charge combustion controlis performed when necessary. In this mode, the throttle valve 23 isopened and closed in accordance with the load applied to the engine 1 bysupplying the necessary amount of air-fuel mixture to produce the powerrequired by the engine 1.

In step 118, the ECU 30 determines whether the present pressuredifference DPBK is greater than the negative pressure value tKPBKSO atwhich the uniform charge combustion brake control is terminated (e.g.,350 mmHg). In most cases, the pressure difference DPBK is still equal toor smaller than the negative pressure value tKPBKSO. In such cases, theECU 30 proceeds to steps 112, 113. This causes the engine 1 tocompletely enter uniform charge combustion. Thus, the throttle valve 23is closed to communicate negative pressure to the brake booster 71.

In step 114, if the execution flag XBKDJ is set at one indicatingexecution of uniform charge brake control (and indicating that uniformcharge combustion is in progress), the ECU 30 proceeds to step 119. Atstep 119, the ECU 30 determines whether the present pressure differenceDPBK is greater than the negative pressure value tKPBKSO at which theuniform charge combustion brake control is terminated (e.g., 350 mmHg).If the pressure difference DPBK is still not greater than the negativepressure, the ECU 30 repeats steps 117, 112, 113 and continues uniformcharge combustion.

When sufficient negative pressure is communicated to the brake booster71 as a result of the uniform charge combustion mode causes the pressuredifference DPBK to become greater than the negative pressure valuetKPBKSO, at which the uniform charge brake control is terminated, theECU 30 proceeds to step 120. The ECU 30 proceeds to step 120 from eitherstep 119 or step 118 (in most cases from step 119). At step 120, the ECU30 determines that there is no need to further produce negative pressureand sets the execution flag XBKDJ to zero. In step 121, the ECU 30discontinues the uniform charge combustion mode and enters thestratified charge combustion mode. The ECU 30 then carries out steps112, 113 and terminates subsequent processing.

In the negative pressure control routine, the pressure difference DPBKis computed from the atmospheric pressure PA and the brake boosterpressure PBK. The closing control of the throttle valve 23 (negativepressure guaranteeing control) is executed when the pressure differenceDPBK is smaller than the negative pressure value tKPBKL, which initiatesstratified charge brake control, or smaller than the negative pressurevalue tKPBKLS, which initiates uniform charge combustion control. If thevehicle speed SPD is lower than the predetermined speed, a negativepressure that guarantees control for stratified charge combustion isprovided. If the vehicle speed SPD is equal to or higher than thepredetermined speed, a negative pressure guaranteeing control foruniform charge combustion is provided.

In addition, it is determined whether the negative pressure thatactuates the brake booster 71 is sufficient (steps 104, 115, etc.). Whenit is determined that the negative pressure is insufficient, the angleclosing control of the throttle valve 23 is carried out. The closing ofthe throttle valve 23 increases the negative pressure (reduces thepressure) and guarantees the operation of the brake booster 71.

To determine whether it is necessary to produce negative pressure forthe actuation of the brake booster 71, the difference between theatmospheric pressure PA and the brake booster pressure PBK, which isdetected by the pressure sensor 63, is computed to obtain the pressuredifference DPBK. If the pressure difference DPBK is smaller than eitherthe negative pressure value tKPBKL, at which stratified charge brakecontrol is initiated, or the negative pressure value tKPBKLS, at whichthe uniform charge brake control is initiated, the closing control ofthe throttle valve 23 (negative pressure ensuring control) is carriedout.

Thus, as shown in FIG. 10, when traveling at a high altitude, thedecrease in the atmospheric pressure PA causes the brake boosterpressure PBK to be lower than when traveling at a low altitude.Accordingly, the brake booster pressure PBK may be low while the actualnegative pressure for actuating the brake booster 71 is insufficient.However, in this embodiment, the closing control of the throttle valve23 is executed to produce negative pressure when the pressure differenceDPBK, and not the brake booster pressure PBK, is smaller than thereference value (negative pressure value tKPBKL for initiatingstratified charge brake control or the negative pressure value tKPBKLSfor initiating uniform charge brake control). This always guaranteessufficient negative pressure for the actuation of the brake booster 71even when the atmospheric pressure PA fluctuates such as when travelingat high altitudes.

Furthermore, the closing control of the throttle valve 23 is executed ifthe pressure difference DPBK is smaller than the reference value(negative pressure value tKPBKL for initiating stratified charge brakecontrol or negative pressure value tKPBKLS for initiating uniform chargebrake control), and the closing control is terminated if the pressuredifference DPBK becomes greater than a larger reference value (negativepressure value tKPBKO for terminating stratified charge brake control ornegative pressure value tKPBKSO for terminating uniform charge brakecontrol). In other words, the reference value has a hysteresis. Thisprevents hunting caused by the pressure difference DPBK becoming smallerthan the reference value and then equal to or greater than the referencevalue in a repetitive manner. Repetitive execution and non-execution ofthe closing control does not take place.

Although the opening area of the intake passage is narrowed to producenegative pressure, an electronically controlled throttling mechanismthat includes the throttle valve 23 and the step motor 22 is employed asa means to guarantee negative pressure. Thus, conventional devices areused to produce negative pressure. This lowers costs.

In this embodiment, when increasing the throttle closing angle TRTCBK,the throttle closing angle TRTCBK is renewed by adding the presently setclosing angle compensation value b to the throttle closing angle TRTCBKof the previous cycle. The closing angle compensation value a is set ata large value if the pressure difference DPBK is much smaller than thenegative pressure value tKPBKO for terminating stratified charge brakecontrol. Therefore, as shown in FIG. 11, the closing speed is highimmediately after the initiation of the closing control. This readilyguarantees negative pressure.

If the pressure difference DPBK approaches the negative pressure valuetKPBKO for terminating stratified charge brake control, the closingangle compensation value a is set at a small value. Thus, when a certaintime elapses after starting the closing control, the closing speeddecreases. This suppresses. As a result, overshooting of the closingaction when negative pressure is sufficient is prevented. Accordingly,starvation of intake air is avoided. This prevents an undesirablecombustion state.

The injection timing control executed during stratified chargecombustion negative pressure control (stratified charge combustion brakecontrol) will now be described. The fuel injection control is executedto prevent the air-fuel ratio from becoming rich when performingstratified brake control. FIG. 8 shows a flowchart of a main routine ofthe fuel injection control executed by the ECU 30.

When entering the main routine, the ECU 30 first reads various detectingsignals such as the degree of acceleration pedal depression ACCP and theengine speed NE in step 201.

In step 202, the ECU 30 determines whether stratified charge combustionis being performed. If the stratified charge combustion is not beingperformed, the ECU 30 temporarily terminates subsequent processing. Ifthe stratified charge combustion is being performed, the ECU 30 proceedsto step 203 and computes the target fuel injection amount TAU.

In step 204, the ECU 30 computes the basic fuel injection timing AINJ(in relation with when the piston is located at the top dead centerposition). In step 205, the ECU 30 computes the target ignition timingSA. In step 206, the ECU 30 computes the basic throttle opening areaTRT. In step 207, the ECU 30 computes the target EGR opening area EGRT.

After obtaining the parameters, the ECU 30 determines whether the flagXBKIDL indicating the execution of the stratified charge brake controlis set at one. If the execution flag XBKIDL is set at one, the ECU 30proceeds to step 209 and computes the fuel injection timing compensationangle KAINJ based on the present throttle closing angle TRTCBK. The ECU30 refers to a map shown in FIG. 9 to compute the injection timingcompensation angle KAINJ. In other words, if the throttle closing angleTRTCBK indicates a value of zero, the injection timing compensationangle KAINJ is set at zero. As the value of the throttle closing angleTRTCBK becomes greater, the fuel injection compensation value KAINJ isset at a larger value (toward advancement).

If the execution flag XBKIDL is not set at one but set at zero, the ECU30 proceeds to step 210 since there is no need to compensate theinjection timing. At step 210, the ECU 30 sets the injection timingcompensation angle KAINJ to zero.

The ECU 30 proceeds to step 211 from either step 209 or step 210 andadvances the basic injection timing AINJ by adding the injection timingcompensation angle KAINJ. The obtained value is set as the final targetinjection timing AINJE. At step 212, the parameters are reflected in thestratified charge combustion. Subsequent processing is then temporarilyterminated.

In the main routine, the closing control of the throttle valve 23 iscarried out while performing stratified charge combustion if thestratified charge brake control is executed. In this case, the targetinjection timing AINJE is advanced from the basic injection timing AINJby the injection timing compensation angle KAINJ. This prevents theair-fuel ratio from becoming rich.

In the main routine, the closing control of the throttle valve 23 iscarried out while performing stratified charge combustion during thestratified charge brake control. In this state, the closing of thethrottle valve 23 causes the air-fuel ratio to become rich since theintake air amount decreases, as shown by the dotted line in FIG. 12. Ifnormal fuel injection and ignition is performed with the air-fuel ratioin a rich state, this may result in an undesirable combustion state.However, in this embodiment, the target injection timing AINJE isadvanced from the basic injection timing AINJ by the injection timingcompensation angle KAINJ, as shown by the double-dotted line in FIG. 12.This guarantees the normal air-fuel ratio during ignition. As a result,the air-fuel ratio is prevented from becoming rich. This prevents anundesirable combustion state.

In the preferred and illustrated embodiment, the firing timing is notaltered. Therefore, fluctuation of the engine torque, which wouldotherwise be caused by changing of the firing timing, is avoided.

A second embodiment of the present invention will hereafter bedescribed. The differences from the first embodiment will mainly bediscussed below, and like or the same reference numerals are given tothose components that are like or the same as the correspondingcomponents of the first embodiment.

In the first embodiment, when the closing control of the throttle valve23 is carried out, the target injection timing AINJE is advanced fromthe basic injection timing AINJ by the injection timing compensationangle KAINJ. This embodiment is different from the first embodiment inthat the firing timing is delayed.

FIG. 13 is a flowchart showing a part of a main routine of a firingtiming control executed by the ECU 30.

When entering the routine, the ECU executes the process of steps 201 to207 as in the first embodiment. At step 301, the ECU 30 determineswhether the flag XBKIDL indicating the execution of the stratifiedcharge brake control is set at one.

If the execution flag XBKIDL is set at one, the ECU 30 proceeds to step302 and computes the firing timing compensation angle KSA based on thepresent throttle closing angle TRTCBK. The ECU 30 refers to a map shownin FIG. 14 to compute the firing compensation angle KSA. In other words,if the throttle closing angle TRTCBK indicates a value of zero, thefiring timing compensation angle KSA is set at zero. As the value of thethrottle closing angle TRTCBK becomes greater, the firing compensationvalue KSA is set at a larger value (causing a greater delay).

If the execution flag XBKIDL is not set at one but set at zero, the ECU30 proceeds to step 303 since there is no need to compensate the firingtiming. At step 303, the ECU 30 sets the firing timing compensationangle KSA to zero.

The ECU 30 proceeds to step 304 from either step 302 or step 304 anddelays the basic firing timing SA by subtracting the firing timingcompensation angle KSA. The obtained value is set as the final targetfiring timing SAE. At step 305, the parameters are reflected in thestratified charge combustion. Subsequent processing is then temporarilyterminated.

In the main routine, the closing control of the throttle valve 23 iscarried out while performing stratified charge combustion if thestratified charge brake control is executed. In this case, the targetfiring timing SAE is delayed from the basic firing timing SA by thefiring timing compensation angle KSA.

In the main routine, the closing control of the throttle valve 23 iscarried out while performing stratified charge combustion during thestratified charge brake control. In this state, the closing of thethrottle valve 23 causes the air-fuel ratio to become rich since theintake air amount decreases, as shown by the dotted line in FIG. 15. Ifnormal fuel injection and ignition is performed with the air-fuel ratioin a rich state, this may result in an undesirable combustion state.However, in this embodiment, the target firing timing SAE is delayedfrom the basic firing timing SA by the firing timing compensation angleKSA, as shown in FIG. 15.

At the times of firing corresponding to the delayed firing timing, theair fuel ratio in the vicinity of the plug 10 is substantially the sameas that of the normal state. This prevents the air-fuel ratio frombecoming rich. This prevents an undesirable combustion state.

Although only two embodiments of the present invention have beendescribed so far, it should be apparent to those skilled in the art thatthe present invention may be embodied in many other specific formswithout departing from the spirit or scope of the invention. Moreparticularly, the present invention may be modified as described below.

(1) In the illustrated embodiment, an electronically controlled throttlemechanism is used as the negative pressure producing means. The throttlemechanism includes the throttle valve 23 arranged in the intake duct 20,and the step motor 22 serving as an actuator for opening and closing thethrottle valve 23. However, an idle speed control (ISC) mechanism may beused as the negative pressure producing means. Such an ISC mechanismincludes an idle speed control valve arranged in an intake passage thatbypasses the throttle valve 23 and an actuator for opening and closingthe control valve.

The EGR mechanism 51 provided with the EGR valve 53 and other parts mayalso be employed as the negative pressure producing means.

The negative pressure mechanism may also be constructed by combiningthese.

(2) The present invention is applied to the cylinder injection typeengine 1 in the illustrated embodiment. The present invention may alsobe applied to an engine that performs stratified charge combustion andweak stratified charge combustion. For example, the present inventionmay be applied to an engine that injects fuel beneath the intake valves6a, 6b provided in the associated intake ports 7a, 7b. The presentinvention may also be applied to an engine that injects fuel directlyinto the cylinder bores (combustion chambers 5) from injection valvesarranged near the intake valves 6a, 6b. As another option, the presentinvention may be applied to an engine that does not perform stratifiedcharge combustion.

(3) In the illustrated embodiment, helical type intake ports areemployed to produce swirls. However, the swirls do not necessarily haveto be produced. In such case, parts such as the swirl control valve 17and the step motor 19 may be eliminated.

(4) The present invention is applied to a gasoline engine in theillustrated embodiment. However, the present invention may also beapplied to other types of engines such as diesel engines.

(5) In the illustrated embodiment, the atmospheric pressure PA isdetected by the intake pressure sensor 61. However, an atmosphericpressure sensor may be provided to detect the atmospheric pressure.

Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive and the invention is not to be limitedto the details given herein, but may be modified within the scope of theappended claims.

What is claimed is:
 1. An apparatus for controlling brake force of a vehicle movable based on rotation of an engine with a plurality of cylinders, each of said cylinders having a combustion chamber that receives fuel from a fuel injector and air from an air intake passage, wherein said air and fuel are mixed and combusted in the combustion chamber, wherein said engine selectively performs a stratified charge combustion and a uniform charge combustion, and wherein said stratified charge combustion mode is selected to increase at least the amount of the air supplied to the engine and improve a combusting state of the engine, said apparatus comprising:a brake booster for increasing said brake force in accordance with negative pressure applied thereto; means for restricting airflow in the air intake passage to generate the negative pressure that is supplied to the brake booster; means for actuating the restricting means to apply the negative pressure to the brake booster when the magnitude relating to the pressure in the brake booster is smaller than a predetermined value; a fuel injector for directly injecting the fuel into the cylinder to set the engine to perform the stratified charge combustion; means for measuring a magnitude relating to the pressure applied to the brake booster; means for detecting the operation state of the engine; means for computing a basic injection timing of the fuel injector based on the detected operation state of the engine; and means for controlling the fuel injector based on the computed basic injection timing, said control means including means for advancing the basic injection timing to stabilize the stratified charge combustion when the stratified charge combustion is performed and the negative pressure is applied to the brake booster.
 2. The apparatus as set forth in claim 1, wherein said advancing means advances the basic injection timing in accordance with amount of the airflow restricted by the restricting means.
 3. The apparatus as set forth in claim 2, wherein said measuring means includes:a first sensor for detecting a first magnitude of the pressure in the brake booster; a second sensor for detecting a second magnitude of air pressure; and first computing means for computing a difference between the first magnitude and the second magnitude.
 4. The apparatus as set forth in claim 1, further comprising:an air intake passage for supplying air to the engine; a valve disposed in the air intake passage to control airflow rate therein; and an actuator for actuating the valve, said actuator forms the generating means in cooperation with the valve.
 5. The apparatus as set forth in claim 4, wherein said valve includes a throttle valve.
 6. The apparatus as set forth in claim 5, further comprising:first calculating means for calculating an opening size of the throttle valve based on the operation state of engine; second calculating means for calculating a correction value that corrects the opening size of the throttle valve calculated by the first calculating means; and said control means actuating the actuator based on the calculated correction value.
 7. The apparatus as set forth in claim 4, wherein said air intake passage has a diverged passage and wherein said valve includes an idle speed control valve in the diverged passage.
 8. An apparatus for controlling brake force of a vehicle movable based on rotation of an engine with a plurality of cylinders, each of said cylinders having a combustion chamber that receives fuel from a fuel injector and air from an air intake passage, wherein said air and fuel are mixed and combusted in the combustion chamber, wherein said engine selectively performs a stratified charge combustion and a uniform charge combustion, and wherein said stratified charge combustion mode is selected to increase at least the amount of the air supplied to the engine and improve a combusting state of the engine, said apparatus comprising:a brake booster for increasing said brake force in accordance with negative pressure applied thereto; means for restricting airflow in the air intake passage to generate the negative pressure that is supplied to the brake booster; means for actuating the restricting means to apply the negative pressure to the brake booster when the magnitude relating to the pressure in the brake booster is smaller than a predetermined value; a fuel injector for directly injecting the fuel into the cylinder to set the engine to perform the stratified charge combustion; means for measuring a magnitude relating to the pressure applied to the brake booster; means for detecting an operation state of the engine; first computing means for computing a basic injection timing of the fuel injector based on the detected operation state of the engine; first control means for controlling the fuel injector based on the computed basic injection timing; means for igniting the fuel in each cylinder; second computing means for computing a basic igniting timing based on the computed operation state of the engine; second control means for controlling the basic igniting timing based on the computed basic igniting timing, said second control means including means for retarding the basic igniting timing to stabilize the stratified charge combustion when the stratified charge combustion is performed and the negative pressure is applied to the brake booster.
 9. The apparatus as set forth in claim 8, wherein said retarding means retards the basic injection timing in accordance with amount of the airflow restricted by the restricting means.
 10. The apparatus as set forth in claim 9, wherein said measuring means includes:a first sensor for detecting a first magnitude of the pressure in the brake booster; a second sensor for detecting a second magnitude of air pressure; and first computing means for computing a difference between the first magnitude and the second magnitude.
 11. The apparatus as set forth in claim 8, further comprising:an air intake passage for supplying air to the engine; a valve disposed in the air intake passage to control airflow rate therein; and an actuator for actuating the valve, said actuator forms the generating means in cooperation with the valve.
 12. The apparatus as set forth in claim 11, wherein said valve includes a throttle valve.
 13. The apparatus as set forth in claim 12, further comprising:first calculating means for calculating an opening size of the throttle valve based on the operation state of engine; second calculating means for calculating a correction value that corrects the opening size of the throttle valve calculated by the first calculating means; and said control means actuating the actuator based on the calculated correction value.
 14. The apparatus as set forth in claim 12, wherein said air intake passage has a diverged passage and wherein said valve includes an idle speed control valve in the diverged passage.
 15. The apparatus as set forth in claim 13, further comprising:means for detecting an operation state of the engine; first calculating means for calculating an opening size of the throttle valve based on the operation state of engine; second calculating means for calculating a correction value that corrects the opening size of the throttle valve calculated by the first calculating means; and said control means actuating the actuator based on the calculated correction value.
 16. The apparatus as set forth in claim 4, wherein said air intake passage has a diverged passage and wherein said valve includes an idle speed control valve. 